SONET/SDH networks have since their introduction in the early 1990s achieved widespread acceptance and widespread usage. The networks transmit data by encoding data into well defined frame structures and then transmitting the data in the frame in a predetermined serial fashion.
The introduction of the SONET/SDH standards allowed network operators to assume a reasonable degree of interoperability between different vendors and thus the standards are used almost exclusively for all fibre-based broadband networks. However, not unexpectedly, a medium sized operator may wish to operate a network based on the SONET or SDH standards with several geographically dispersed networks. For example, an operator may have a network covering a city which it wishes to interconnect with a similar network covering a distant second city, For such an operator, the provisioning of a dedicated SONET or SDH fibre link between the two cities may be prohibitively expensive and/or not justifiable in terms of potential bandwidth usage. A typical reaction to this problem is to take the traditional business model of a “leased line”. In this business model, the medium sized operator approaches a third party “carrier” to buy bandwidth on a fibre link which already exists between the two cities. In principal, this approach should be effective. However, a careful analysis shows that there are significant drawbacks with the prior art implementations of such a “leased line” approach.
What is desired is that the interconnection between the networks is entirely transparent so that it appears as if the SONET/SDH network elements in the two regions are directly connected over fibre.
Unfortunately, present solutions do not economically meet this need.
The first option to interconnect the two regional networks is to use so-called “dark fibre”. This is simply a dedicated fibre which is leased in its entirety from the carrier. This provides the idealised solution but is as mentioned above, likely to be prohibitively expensive depending on the length of the fibre and the likely bandwidth utilisation.
In view of this, other prior art solutions have been proposed which attempt to carry an existing SONET/SDH data stream within a carrier's SONET/SDH network.
A first such solution is to use a digital wrapper on a dense wavelength division multiplexed (DWDM) wavelength on the fibre. However, there are many digital wrapper schemes; both proprietary and standard (as defined in ITU-TG.709). The schemes add a new non-SONET wrapper to the clients signal for use with DWDM systems. A significant problem with this approach is that the digital wrapper does not include a multiplexing scheme which means that the carrier cannot switch or cross-connect the signal electrically. Therefore the carrier must provision a new layer of wavelength or photonic switching to allow the signal to be routed through the carrier's network. This makes this solution expensive.
The second such solution is to carry the SONET/SDH payload in the carriers payload and to remap the operator's overhead to a different position in the carrier's overhead. It will be noted, that this is not fully transparent since it does not allow for proprietary use of the SONET/SDH overhead by the client network operator. In practice this is a serious problem. Furthermore, pointer processing within the carrier network disrupts the client operators signal clock content. This means that the geographically separated networks cannot operate fully transparently with synchronised clocks.
A third such solution is to map the operator's signal into the carriers SONET/SDH signal using real (or contiguous) concatenation. However, real concatenation is designed primarily to allow the carrying of high bandwidth signals (higher than the standard predefined SDH/SONET structures) using SONET/SDH. It will be noted that the present need is to carry a signal in one SONET/SDH structure using another SONET/SDH structure. Thus the requirement to solve this problem is to fit one structure inside another identically-sized structure and yet retain additional data such as the original overhead data. Thus using real concatenation using the standard concatenation sizes typically wastes at least nearly half the bandwidth of the real concatenated structure since to maintain all the desired information it is necessary to choose a real concatenated structure which is next largest in size than the operators structure which is to be carried by the carriers structure.
In addition to this massive over-provisioning resulting in large quantities of bandwidth being purchased from the carrier but remaining unused, real concatenation requires pointer processing to be carried out which affects the error count maintained in the overhead which in turn means that the process is not fully transparent.
Thus none of the prior art solutions (other than the prohibitively expensive dark fibre option) are able to carry the client operator signal transparently in the sense that the overhead payload and clock content remain entirely unaltered on receipt at the operators second regional network.
With this technique the client operator can build a SONET/SDH network without having to build its own entire transport network, by using facilities leased from a carrier. Such a facility then permits the proprietary use of SONET/SDH overhead, separation between the carrier and client network operators clock domains, separation of the OAM data communications (SONET/SDH DCN) domains and reduced inter-operability requirements.